The main Syrian opposition group says it has turned down invitations to visit Moscow and Washington.
In a statement issued on February 22, the Syrian National Coalition said it was taking the action to protest the lack of an international response to what it called the systematic destruction of the ancient city of Aleppo by Syrian missile strikes.
The coalition said it had also suspended participation in the friends of Syria conference of international powers due in Rome next month to protest the attacks it said have killed hundreds of civilians.
The coalition singled out Russia for its continued support for the regime of Syrian leader Bashar al-Assad.
Earlier the coalition announced it will establish a government for "liberated areas." A coalition spokesman said officials for the proposed government would be selected at a meeting in Istanbul on March 2.
The announcement was made February 22 in Cairo following two days of talks by the Syrian National Coalition.
The coalition also said it was willing to negotiate a peace deal to end the country's civil war, but insisted Assad must step down and not be a part of any settlement.
On the ground inside Syria, there have been reports of fresh air strikes on Aleppo.
The Syrian Observatory for Human Rights said at least 29 people had been killed and 150 wounded.
The observatory told the dpa news agency that bodies were still buried under rubble.
That report could not be independently verified.
Earlier, international peace envoy Lakhdar Brahimi said attacks in Damascus on February 21 had left about 100 people dead -- substantially more than a previous toll of 60 people -- and wounded another 250.
Describing the attack as a "war crime," the UN-Arab League envoy added in a statement: "Nothing could justify such horrible actions that amount to war crimes under international law."
European Union foreign policy chief Catherine Ashton also condemned the Damascus attack.
Based on Reuters and dpa reporting